The present invention relates to a focus detecting apparatus for a camera and, more particularly to a focus detecting apparatus with a charge storage type image sensor to control focusing on the basis of an output from the image sensor.
A so-called TTL focus detecting apparatus using a focus detection scheme for detecting light rays passing through a photographing lens in a camera has an image sensor with photoelectric conversion elements aligned in an array. In to a contrast detection system, an optical image of an object which is focused on the image sensor is subjected to focus detection in accordance with acuity of the image. In to a defocusing detection system, two light components passing through two different regions of an exit pupil of a photographing lens are received by a pair of photoelectric conversion elements, and the focusing state is detected by a deviation between two images formed by the elements.
These focus detection systems utilize luminance distribution (object pattern) of the object. The photoelectric conversion part of either system comprises a charge storage type image sensor such as a CCD or MOS image sensor. A time serial signal from the sensor is arithemtically operated to perform focus detection.
As is well known, since the luminance level of the object varies in a wide range in a conventional focus detection apparatus for a camera, the charge storage time of the image sensor must be controlled in accordance with the luminance level of the object to provide a wider dynamic range of the image sensor.
In a conventional focus detection apparatus described in U.S. Pat. No. 4,410,258, an object luminance level monitor sensor is arranged near a focus detection sensor to control the charge storage time of the focus detection sensor in such a manner that an average value of the photoelectric conversion output from the focus detection sensor is set to be a predetermined value. In another conventional focus detection apparatus described in U.S. Pat. No. 4,329,577, a peak value of a time serial signal from a focus detection sensor is detected and discriminated to determine whether or not it exceeds a predetermined level. When the peak value exceeds the predetermined level, the charge storage time of the focus detection sensor is shortened. In the following description, the former type of apparatus is called an average type focus detection apparatus, and the latter type is called a peak type focus detection apparatus.
These conventional focus detection apparatuses have the following drawback. This will be described with reference of the object luminance distribution in FIGS. 1A to 1D and FIGS. 2A to 2D.
FIGS. 1A and 2A respectively show luminance distributions of outputs of the conventional average and peak type focus detection apparatuses, which correspond to a black stripe on a white background. FIGS. 1B and 2B show respectively luminance distributions of outputs of the conventional average and peak type focus detection apparatuses, which correspond to a white stripe on a black background. FIGS. 1C and 2C show respectively luminance distributions of high-frequency, high-contrast outputs from the average and peak type focus detection apparatuses. FIGS. 1D and 2D respectively show luminance distributions of low-frequency, low-contrast outputs of the average and peak type focus detection apparatuses. In each graph in FIGS. 1A to 1D and FIGS. 2A to 2D, the photoelectric conversion output is plotted along the ordinate, and the photoelectric conversion element array is plotted along the abscissa.
Referring to FIGS. 1A to 1D, the average value of the photoelectric conversion output in the average type focus detection apparatus is controlled to be a predetermined value Va. The charge storage time is excessively prolonged as shown in FIG. 1B. In this case, a photoelectric conversion output of the white background is excessively large and is not suitable for the input range of the subsequent processing circuit. In FIG. 1C, sufficient focus detection precision can be obtained due to the high-frequency, high-contrast object although the charge storage time is short. However, since the predetermined value Va is provided, the charge storage time is excessively prolonged. In FIG. 1D, the charge storage time should be prolonged for the low-frequency, low-contrast object but cannot be prolonged due to the predetermined value. As an example for the case in FIG. 1B, streets flooded with neon lights have a distinctive luminance distribution. However, since the background black level has a low illuminance, the charge storage time is prolonged, and dark current noise is increased, thereby degrading response time of the focus detection signal and focus detection precision. In order to eliminate these drawbacks, when the charge storage time is set to decrease the average level, noise (e.g., a quantization error of A/D conversion) of a focus detection arithmetic system is increased. As a result, the overall S/N ratio is decreased, and high focus detection precision cannot be obtained. It is therefore difficult to optimally set the average level.
In the peak type focus detection apparatus in FIGS. 2A to 2D, it is difficult to detect a peak value Vp within the charge storage time of the photoelectric conversion element, so that the peak value is detected after the photoelectric conversion output is read out. For example, in the luminance distribution (FIG. 1B) representing a saturated state of the object, i.e., an input exceeding the input range of the subsequent processing circuit, the charge storage time must be shortened and the photoelectric conversion output must be repeatedly read out until the state of FIG. 2B is obtained, i.e., until the peak value reaches the predetermined level Vp. The arithmetic operation must then be repeated a plurality of times until the peak value reaches the predetermined level. Therefore, the focus detection signal cannot be obtained at high speed. In particular, when the object has a low luminance level, i.e., when the charge storage time is prolonged, the focusing response time is greatly prolonged.
The conventional average and peak type focus detection apparatuses have the above drawbacks. Charge storage time is needlessly prolonged in accordance with a given object luminance distribution, thereby prolonging the focusing response time and degrading focu detection precision.